1. Technical Field of the Invention
The present invention relates generally to digital image processing, and specifically to the reduction of color aliasing artifacts in decompressed digital images.
2. Description of Related Art
Demosaicing refers to the process of interpolating colors of a digital image obtained from an image sensor fitted with a color filter array (CFA). One such CFA is described in U.S. Pat. No. 3,971,065 to Bayer (hereinafter referred to as Bayer), which is hereby incorporated by reference. In the Bayer CFA, each pixel sees only one color: red, green or blue. To obtain all three primary colors at a single pixel, it is necessary to interpolate colors from adjacent pixels. This process of interpolation is called demosaicing. Demosaiced images frequently exhibit color aliasing artifacts (distortion) due to the inherent under-sampling of color on an image sensor fitted with a CFA.
Due to the large amount of memory necessary for storing demosaiced images, many applications utilize image compression techniques that represent the demosaiced images with less data in order to save storage costs or reduce transmission time. The greater the compression, the more approximate the decompressed image will be. Compression can be performed using a lossless compression technique or using a lossy technique, such as JPEG (Joint Photographic Experts Group). A discussion of the JPEG technique can be found in: W. Pennebaker and J. Mitchell, xe2x80x9cJPEG: Still Image Data Compression Standard,xe2x80x9d New York: Van Nostrand Reinhold, 1993, which is hereby incorporated by reference.
Previous methods for reducing color aliasing have been designed for discrete cosine transform based JPEG image compression processes (see Pennebaker and Mitchell). However, if the demosaiced image is compressed using wavelets, such as in the JPEG 2000 standard, which is discussed in ISO/IEC 15444-1:2000, Coding of Still Pictures: JPEG 2000, Part 1, which is hereby incorporated by reference, the discrete cosine transform color aliasing reduction methods are not suitable for use with wavelet-based compression.
A system and method is provided for processing a demosaiced image using a color aliasing artifact reduction (CAAR) algorithm in order to reduce color aliasing artifacts. The CAAR algorithm computes the L level wavelet transform for the demosaiced color planes R, G and B. Thereafter, the CAAR algorithm estimates the correct color value at each pixel location for the colors not associated with that pixel location. For example, to determine the green value at red pixel locations, the CAAR algorithm performs an inverse wavelet transform using the green approximation signal and the red detail signals. This process is repeated for each of the colors (e.g., green values at blue pixel locations, red values at green pixel locations, etc.). In addition, the CAAR algorithm performs an inverse wavelet transform on each of the color planes themselves, so that the pixel values of the color associated with each pixel location are not altered. Thereafter, the inverse wavelet transform of each color plane is combined with the inverse wavelet transform of each of the estimated color values for that color plane to produce correlated R, G and B color planes. It is these correlated R, G and B color planes that may later be compressed using a wavelet-based image compression method, such as the JPEG 2000 standard.